The present invention relates generally to a process for extracting metal values from ores, and more specifically, to a process for extracting non-ferrous metal values from oxide ores.
Many types of geological ore deposits are known which contain naturally occurring aggregate or combination of minerals from which one or more non-ferrous metals such as silver, copper, lead, zinc, gold, nickel, magnesium, or chromium can be extracted. From a metallurgical standpoint, a few of the naturally occurring ores contain non-ferrous metals in their metallic or native state. However, the great majority of ores contain non-ferrous metals in combination with other elements. The most common types of the latter ores are (a) oxide ores which can include oxides, carbonates and silicates of the non-ferrous metals, and (b) sulfide ores which can include sulfides, selenides, tellurides, arsenides, and/or antimonides of the non-ferrous metals. (J. L. Bray, "Non-Ferrous Production Metallurgy", John Wiley & Sons, New York, N. Y., 1947, pp 19-20).
Conventionally, ores in which the non-ferrous metal values are in the form of the sulfides have been extensively mined due to the ready availability of these ores and the relatively large proportion of the non-ferrous metals which may be economically recovered from the sulfide ores. However, these high-grade sulfide containing mineral ores are rapidly being depleted, and industry is being forced to consider other classes of ores, wherein the metal content may not be as readily recoverable as the sulfide type ores.
One area of particular metallurgical interest is ores of the oxide type, some of which are believed to contain appreciable percentages of valuable non-ferrous metal values in various forms. Although these ores can contain attractive non-ferrous metal values, they have not been successfully processed on a large commercial scale. This is due, partially to the relative abundance of the sulfide ores, partially to the fact that the non-ferrous metals are often associated with other material which render their extraction expensive, and partially because the present methods of concentrating sulfide ores have not been successfully adapted to include oxide type ores.
A number of processes have been proposed for the extraction of non-ferrous metal values from oxide ores. Historically, the oldest method of separating non-ferrous metal values from oxide ores is by smelting the mined ore or concentrates of it. However, the smelting process can result in the release of undesirable contaminants into the environment thus creating a serious air pollution problem.
Another process for removing non-ferrous metal values from ores is the method known as "cyanidation". The cyanidation process consists of leaching mined and crushed ore with dilute solutions of sodium cyanide and oxygen, thereby dissolving the metal as the complex cyanide ion, and then precipitating the metal by the addition of zinc or aluminum to the cyanide solution. Attempts to cyanide leach a non-ferrous metal, such as silver, from argentiferous oxide ores on a commercial scale can result in uneconomical metal recovery.
A third method of processing non-ferrous metal values from ores is chloride-volatilization. This process involves the isolation of non-ferrous metals from unwanted material by formation of metal chlorides with a suitable chloridizing agent and volatilization of these metal chlorides from the unwanted material. This process is not economically attractive, due to its high energy requirements.
It is therefore desired to develop a process for extracting non-ferrous metals from oxide ore. The method should provide a high percentage recovery of the desired metal values from low-grade oxide ore, while reducing the environmental impact and high energy cost associated with conventional procedures.
These and other advantages are achieved by the present process.
For the purposes of the specification, the term oxide ores refers to minerals which contain any amount of metal oxides, carbonates, silicates, or mixtures thereof. It can also include naturally occurring sulfide ores which have been converted to the oxide form by suitable treatment, such as heating. It will be understood that some oxide ores may contain only minor amounts of non-ferrous metal oxides, and may not be subject to economical recovery by the present process. It should also be understood that the non-ferrous metal recovered from oxide ores may initially be in a form other than as an oxide, carbonate, silicate, or mixture thereof. However, the non-ferrous metals will generally be dispersed throughout an "oxide" type mineral matrix.